Maxi-K(Ca)calcium-activated potassium channels are encoded by the Slo gene. They are widely expressed in the nervous system and are enriched at presynaptic terminals of neurons. Maxi-K(Ca) channels are likely to influence the pattern of response to external stimulation, the amount and timing of neurotransmitter release. The unitary conductance and other characteristics of Maxi-K(Ca) channels have been reported to differ in different neurons. There is only one Slo gene that is known to be expressed in neurons, but a variety of Slo Proteins can be generated by alternative splicing of RNA. A second gene, termed Slack, has recently been identified and shown to encode potassium channels that are quite distinct from Slo channels. Evidence suggests, however, that Slack channel subunits can interact directly with Slo subunits to generate Maxi-K(Ca)-like channels with conductances and kinetic properties that are intermediate between those of Slack and Slo expressed alone. Experiments in this proposal will determine the cellular and subcellular localization of Slo and Slack channel subunits in neurons, patch clamp experiments will be carried out on presynaptic terminals and postsynaptic somata of neurons in the medial nucleus of the trapezoid body, where Slack is expressed at high levels, to compare native currents with those in cells transfected with these genes. The hypothesis that Slo and Slack proteins form heteromultimers in native neurons will be tested by immunochemical experiments. To test the roles of Slack and Slo channels, homologous recombination will be used to eliminate the expression of the Slack gene from the mouse genome. These studies will lead to an understanding of how electrical activity and synaptic transmission are regulated by calcium-dependent K channels.